1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a printer or the like which has a charging member for charging an image bearing body such as an electrophotographic sensitive body, an electrostatic recording dielectric or the like to predetermined polarity and potential.
More particularly, the present invention relates to an image forming apparatus which adopts, as a charging means for the image bearing body, a contact charging means using charging accelerating particles.
2. Related Art
Conventionally, in an image forming apparatus such as a copying machine, a printer or the like, a latent image formed on an image bearing body such as an electrophotographic sensitive body, an electrostatic recording dielectric or the like is visualized by using a dry developing agent (developer) being granular materials. In recent years, to cope with downsizing, simplification, energy saving and environmental measures of the apparatus, it pays attention to an image forming apparatus which disuses a cleaning container and a waste toner container and can achieve ozoneless operation that generation of ozone due to electrical discharge is decreased and a cleanerless operation.
1. Ozoneless Charging Means
As a means for performing an ozoneless charging process to the image bearing body, a contact charging means using charging accelerating particles is disclosed in Japanese Patent Application Laid-Open Nos. 10-307454 to 10-307459, U.S. Pat. Nos. 6,134,407, 6,081,681 and 6,128,456, and the like.
In this contact charging means, the charging accelerating particles are interposed in a charging nip (charging area, charging part) which is the contact portion of at least both the image bearing body (hereinafter called a photosensitive drum) functioning as the body to be charged and a contact charging member (hereinafter called a charging roller) being generally a conductive/elastic roller caused to be in contact with the photosensitive drum, whereby a direct injection charging mechanism is more predominant than a discharge charging mechanism.
The charging accelerating particles are the conductive particles which aim to assist the charging. For example, various conductive particles, each having the particle diameter 0.1 mm to 5 mm and the volume resistance 1xc3x971012 xcexa9xc2x7cm or less, more desirably 1xc3x971010 xcexa9xc2x7cm or less, such as metal oxide particles (e.g., conductive zinc oxide), other conductive inorganic particles, compounds with organic materials, and the like can be used.
Since the charging accelerating particles exist, the charging roller can be in contact with the photosensitive drum in the charging nip with a speed difference and also be in contact with the photosensitive drum closely through the charging accelerating particles. That is, the charging accelerating particles existing in the charging nip compactly scrape the surface of the photosensitive drum, whereby a charge is directly injected into the photosensitive drum, then, in the charging of the photosensitive drum by the charging roller, the direct injection charging mechanism can be made predominant by the existence of the charging accelerating particles.
Therefore, it is possible to achieve with simple structure low-apply-voltage and ozoneless direct injection charging that high charging efficiency which could not be obtained in conventional roller charging and the like can be obtained, and the surface of the photosensitive drum provided with a charge injection layer can be charged, without using any discharge, to a potential approximately equal to a charging bias voltage applied to the charging roller. Such the direct injection charging is effective for the charging means which performs the direct injection charging process uniformly and ozonelessly to the image bearing body such as the electrophotographic sensitive body, the electrostatic recording dielectric or the like so that the image bearing body has predetermined polarity and potential, in an electrophotographic image forming apparatus and an electrostatic recording image forming apparatus.
The charging roller has a foamed body so that the charging accelerating particles may effectively exist in the charging nip. That is, the charging accelerating particles are maintained within the foams on the surface of the charging roller, whereby an opportunity that the charging roller comes into contact with the photosensitive drum is increased.
Such a charging system not using the discharge can prevent discharge products from being created, whereby abrasion of the photosensitive drum due to the discharge products can be prevented, and a lifetime of the photosensitive drum can be thus prolonged. Besides, as compared with a general charging system using AC voltage, the above charging system is effective for energy saving and cost reduction because the photosensitive drum can be charged by low DC voltage.
2. Cleanerless
The image forming apparatus of cleanerless system has the structure that the cleaning container and the waste toner container for eliminating from the surface of the photosensitive drum a developing agent (hereinafter called toner) slightly remaining on this surface after a developing-agent image was transferred from the photosensitive drum to a recording medium are disused, the transfer residual toner on the photosensitive drum is eliminated therefrom by xe2x80x9cdeveloping simultaneous cleaningxe2x80x9d with a developing means, and the eliminated toner is then retrieved to the developing means and reused.
The developing simultaneous cleaning is the method that the toner slightly remaining on the photosensitive drum after the image was transferred is retrieved by a fog elimination bias (fog elimination potential difference Vback being the potential difference between the DC voltage applied to the developing means and the surface potential on the photosensitive drum) when developing is performed in next and following steps.
According to such a method, since the transfer residual toner is collected (retrieved) by the developing means and used for the developing in the next and following steps, the waste toner can be suppressed, and thus trouble for maintenance can be decreased. Besides, since the cleanerless system is adopted, an advantage in respect of space is great, whereby the image forming apparatus can be greatly downsized.
If the charging means of the photosensitive drum is the contact charging means, it is possible to cause the contact charging member being in contact with the photosensitive drum to once retrieve the transfer residual toner for charging polarity adjusting and then emit the retrieved toner again to the photosensitive drum so that the emitted toner is retrieved by the developing means.
Further, if the charging means of the photosensitive drum performs the direct injection charging process using the charging accelerating particles, it is possible to uniformly charge the photosensitive drum in the image forming apparatus of cleanerless type.
Concretely, the charging accelerating particles are mixed with the developing agent of the developing means. Then, at the developing portion, the charging accelerating particles are supplied from the developing means to the surface of the photosensitive drum together with the toner, and at the transferring portion, only the toner is mainly transferred to the recording medium, and the charging accelerating particles are supplied to the charging nip being the contact portion between the charging roller and the photosensitive drum. Thus, it is possible to uniformly charge the photosensitive drum by the injection charging in the image forming apparatus of cleanerless type.
As above, the structure to supply the charging accelerating particles from the developing means to the charging nip being the contact portion between the charging roller and the photosensitive drum is disclosed in Japanese Patent Application Laid-Open No. 10-307455 and the like.
Incidentally, if the image forming operation is performed by using the image forming apparatus of cleanerless type which adopts, as the charging means for the image bearing body, the contact charging means using the charging accelerating particles, following problems become remarkable as the number of sheets of printing increases.
(1) Defective Charging due to Toner Clogginess within Foams of Charging Roller
As the number of accumulated sheets of printing increases, defective charging occurs. Then, as the number of accumulated sheets of printing further increases, the defective charging becomes remarkable, whereby desired high-fineness image printing becomes impossible.
Besides, when the photosensitive drum is started from the state that it had stopped for a long time, or when the image forming apparatus is intermittently used, the defective charging is remarkable.
The inventor and the like of the present invention pursued the cause of such the defective charging, and found that the defective charging occurs because the toner clogs the foams on the surface of the charging roller being the contact charging member and thus the charging accelerating particles of a sufficient amount necessary for the charging are maintained in the charging nip.
Such the toner clogginess occurs when the transfer residual toner, which was not able to be transferred to the side of a recording medium by a transferring means when a printing operation is performed, on the photosensitive drum being the image bearing body reaches the charging nip and penetrates the foams on the surface of the charging roller. Since the toner which penetrated the foams is distorted and the shape thereof is uneven because it has inequalities on its surface, movement of the toner does not easily occur within the foam itself and between the linked foams, and the inequalities on the surface of the toner are caught by the foam and the foam wall. As above, since the toner does not flow in the foams, the foams which should originally hold the charging accelerating particles are filled with the toner fast if the printing operation continues, whereby the charging accelerating particles can not be held on the surface of the charging roller.
Moreover, in addition to the cause of the defective charging by the transfer residual toner, it was also clarified that the charging roller is filled with the toner when driving of the photosensitive drum starts. The reason why the toner is carried when the driving of the photosensitive drum starts is that a part of the toner existing in the portion (hereinafter called a developing nip) where the photosensitive drum is in contact with the developing roller of the developing means while the drum is stopping is adhered on the drum by surface adsorption force, the adhered toner is then moved to the charging nip by the rotation of the drum, and the moved toner thus reaches the charging roller. Since the toner almost loses the charge by leaving when the apparatus is stopping, such the toner can not be retrieved to the developing roller by using electrical action even if a potential difference is given between the photosensitive drum and the developing roller.
If a separation mechanism is provided between the photosensitive drum and the developing roller, the toner clogginess when the driving of the photosensitive drum starts can be prevented. However, in such a case, a problem that a complicated separation mechanism is necessary and thus it costs is newly caused.
A part of the uncharged toner which reached the charging nip passes this nip by the rotation of the photosensitive drum, but most of the residual toner penetrates the foams of the charging roller in the charging nip. Since the toner which penetrated the foams is distorted and the shape thereof is uneven because it has inequalities on its surface, movement of the toner does not easily occur within the foam itself and between the linked foams, and the inequalities on the surface of the toner are caught by the foam and the foam wall.
Conventionally, the toner of which the shape is uneven is used. For example, a resin, a pigment, a charging control agent and the like are kneaded by a mixing machine, and the kneaded matter is further crushed and sorted, whereby so-called crushed toner of which the shape is uneven can be obtained.
As above, since the toner does not flow in the foams, the foams which should originally hold the charging accelerating particles are filled with the toner fast if the printing operation continues, whereby the charging accelerating particles can not be held on the surface of the charging roller.
The reason why the defective charging becomes remarkable when the photosensitive drum is started from the state that the apparatus had stopped for a long time or when the apparatus is intermittently used is that the large amount of the uncharged toner which can not be electrically retrieved reaches the charging nip.
Further, if the charging roller is filled with the toner, hardness of the charging roller increases fast, whereby a problem that a torque increases and an excessive load is applied to the driving motor for the charging roller is caused.
In addition, if the contact pressure of the charging nip increases and thus frictional heat is caused excessively, a problem that the toner in the foams of the charging roller softens and is fused to the photosensitive drum and the charging roller is caused.
(2) Fog on Drum
In addition to the problem of the defective charging due to the toner clogginess within the foams of the charging roller, a following problem is caused.
That is, the problem that so-called xe2x80x9cfog on drumxe2x80x9d in which the toner is adhered on the photosensitive drum even if any latent image does not exist on the drum in the image forming operation occurs, whereby a transferring material is stained, and the toner causing the fog again reaches the charging nip to cause the defective charging as in the problem (1).
Like the problem (1), particularly, when the photosensitive drum is started from the state that it had stopped for a long time, or when the image forming apparatus is intermittently used, the fog on the photosensitive drum is remarkable.
The inventor of the present invention pursued the cause of such the fog on the drum, and found that the fog occurs because the uncharged toner and the inadequately charged toner exist on the photosensitive drum after it passed the charging nip. Conventionally, the toner of which the shape is uneven is used, and reversal toners such as the uncharged toner, the transfer residual toner and the like which reached the charging nip when the driving of the photosensitive drum starts are not mixed frictionally with the charging accelerating particles in the charging nip uniformly, whereby a charging amount of the toner becomes uneven, and the uncharged toner which is hardly charged and the reversal toner of which the surface is not uniformly charged and which passed the charging nip without sufficient regular charging exist. As a result, the toner adhered on the non-image portion can not be retrieved into the developing means by an electric field between the photosensitive drum and the developing roller, whereby the fog on the drum occurs.
The reason why the fog on the photosensitive drum becomes remarkable when the drum is started from the state that it had stopped for a long time or when the apparatus is intermittently used is that, as described above, the large amount of the uncharged toner reaches the charging roller.
An object of the present invention is to provide an image forming apparatus which prevents defective charging due to stagnation of toner on a charging member.
Another object of the present invention is to provide an image forming apparatus which can stably bear conductive particles on a charging member.
Still another object of the present invention is to provide an image forming apparatus which prevents hardness of a charging member from increasing.
Still another object of the present invention is to provide an image forming apparatus which prevents a fog of toner on an image bearing body.
Still another object of the present invention is to provide an image forming apparatus which can decrease uncharged toner and reversal toner.
Other objects and features of the present invention will become more apparent when reading the following detailed description with reference to the attached drawings.